Autonomic activity in trauma patients based on variability of heart rate and respiratory rate

A Systematic Review of the Relationship Between Traumatic Brain Injury and Disruptions in Heart Rate Variability

Autonomic nervous system dysfunction is increasingly recognized as a common sequela of traumatic brain injury (TBI). Heart rate variability (HRV) is a specific measure of autonomic nervous system functioning that can be used to measure beat-to-beat changes in heart rate following TBI. The objective of this systematic review was to determine the state of the literature on HRV dysfunction following TBI, assess the level of support for HRV dysfunction following TBI, and determine if HRV dysfunction predicts mortality and the severity and subsequent recovery of TBI symptoms. We followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. Two raters coded each article and provided quality ratings with discrepancies resolved by consensus. Eighty-nine papers met the inclusion criteria. Findings indicated that TBI of any severity is associated with decreased (i.e., worse) HRV; the severity of TBI appears to moderate the relationship between HRV and recovery; decreased HRV following TBI predicts mortality beyond age; HRV disturbances may persist beyond return-to-play and symptom resolution following mild TBI. Overall, current literature suggests HRV is decreased following TBI and may be a good indicator of physiological change and predictor of important outcomes including mortality and symptom improvement following TBI.

Targeting hydrogen sulfide and nitric oxide to repair cardiovascular injury after trauma

The systemic cardiovascular effects of major trauma, especially neurotrauma, contribute to death and permanent disability in trauma patients and treatments are needed to improve outcomes. In some trauma patients, dysfunction of the autonomic nervous system produces a state of adrenergic overstimulation, causing either a sustained elevation in catecholamines (sympathetic storm) or oscillating bursts of paroxysmal sympathetic hyperactivity. Trauma can also activate innate immune responses that release cytokines and damage-associated molecular patterns into the circulation. This combination of altered autonomic nervous system function and widespread systemic inflammation produces secondary cardiovascular injury, including hypertension, damage to cardiac tissue, vascular endothelial dysfunction, coagulopathy and multiorgan failure. The gasotransmitters nitric oxide (NO) and hydrogen sulfide (H2S) are small gaseous molecules with potent effects on vascular tone regulation. Exogenous NO (inhaled) has potential therapeutic benefit in cardio-cerebrovascular diseases, but limited data suggests potential efficacy in traumatic brain injury (TBI). H2S is a modulator of NO signaling and autonomic nervous system function that has also been used as a drug for cardio-cerebrovascular diseases. The inhaled gases NO and H2S are potential treatments to restore cardio-cerebrovascular function in the post-trauma period.

Parasympathetic and Sympathetic Monitoring Identifies Earliest Signs of Autonomic Neuropathy

The progression of autonomic dysfunction from peripheral autonomic neuropathy (PAN) to cardiovascular autonomic neuropathy, including diabetic autonomic neuropathy and advanced autonomic dysfunction, increases morbidity and mortality risks. PAN is the earliest stage of autonomic neuropathy. It typically involves small fiber disorder and often is an early component. Small fiber disorder (SFD) is an inflammation of the C-nerve fibers. Currently, the most universally utilized diagnostic test for SFD as an indicator of PAN is galvanic skin response (GSR), as it is less invasive than skin biopsy. It is important to correlate a patient’s symptoms with several autonomic diagnostic tests so as not to treat patients with normal findings unnecessarily. At a large suburban northeastern United States (Sicklerville, NJ) autonomic clinic, 340 consecutive patients were tested with parasympathetic and sympathetic (P&S) monitoring (P&S Monitor 4.0; Physio PS, Inc., Atlanta, GA, USA) with cardiorespiratory analyses, and TMFlow (Omron Corp., Hoffman Estates, Chicago, IL, USA) with LD Technology sudomotor test (SweatC™). This is a prospective, nonrandomized, observational, population study. All patients were less than 60 y/o and were consecutively tested, analyzed and followed from February 2018 through May 2020. P&S Monitoring is based on cardiorespiratory analyses and SweatC™ sudomotor testing is based on GSR. Overall, regardless of the stage of autonomic neuropathy, SweatC™ and P&S Monitoring are in concordance for 306/340 (90.0%) of patients from this cohort. The result is an 89.4% negative predictive value of any P&S disorder if the sudomotor GSR test is negative and a positive predictive value of 90.4% if the sudomotor testing is positive. In detecting early stages of autonomic neuropathy, P&S Monitoring was equivalent to sudomotor testing with high sensitivity and specificity and high negative and positive predictive values. Therefore, either testing modality may be used to risk stratify patients with suspected autonomic dysfunction, including the earliest stages of PAN and SFD. Moreover, when these testing modalities were normal, their high negative predictive values aid in excluding an underlying autonomic nervous system dysfunction.

Wearable respiration monitoring using an in-line few-mode fiber Mach-Zehnder interferometric sensor

Continuous respiratory monitoring is extensively important in clinical applications. To effectively assess respiration rate (RR), tidal volume (TV), and minute ventilation (MV), we propose and experimentally demonstrate a respiration monitoring system using an in-line few-mode fiber Mach-Zehnder interferometer (FMF-MZI), which is the first to introduce in-line MZI into an optimal wearable design for respiration rate and volume monitoring. The optimal linear region of the proposed sensor is analyzed and positioned by a flexible arch structure with curvature sensitivity up to 8.53 dB/m-1. Respiration monitoring results are in good agreement with a standard spirometer among different individuals. The difference in TV estimation is ± 0.2 L, and the overall error of MV estimation is less than 5%.

Autonomic impairment of patients in coma with different Glasgow coma score assessed with heart rate variability

PRIMARY OBJECTIVE

The objective of this study is to assess the functional state of the autonomic nervous system in healthy individuals and in individuals in coma using measures of heart rate variability (HRV) and to evaluate its efficiency in predicting mortality.

DESIGN AND METHODS

Retrospective group comparison study of patients in coma classified into two subgroups, according to their Glasgow coma score, with a healthy control group. HRV indices were calculated from 7 min of artefact-free electrocardiograms using the Hilbert-Huang method in the spectral range 0.02-0.6 Hz. A special procedure was applied to avoid confounding factors. Stepwise multiple regression logistic analysis (SMLRA) and ROC analysis evaluated predictions.

RESULTS

Progressive reduction of HRV was confirmed and was associated with deepening of coma and a mortality score model that included three spectral HRV indices of absolute power values of very low, low and very high frequency bands (0.4-0.6 Hz). The SMLRA model showed sensitivity of 95.65%, specificity of 95.83%, positive predictive value of 95.65%, and overall efficiency of 95.74%.

CONCLUSIONS

HRV is a reliable method to assess the integrity of the neural control of the caudal brainstem centres on the hearts of patients in coma and to predict patient mortality.

Cardiac Autonomic Neuropathy in Diabetes: A Predictor of Cardiometabolic Events

Autonomic nervous system (ANS) imbalance manifesting as cardiac autonomic neuropathy in the diabetic population is an important predictor of cardiovascular events. Symptoms and signs of ANS dysfunction, such as resting heart rate elevations, diminished blood pressure responses to standing, and altered time and frequency domain measures of heart rate variability in response to deep breathing, standing, and the Valsalva maneuver, should be elicited from all patients with diabetes and prediabetes. With the recognition of the presence of ANS imbalance or for its prevention, a rigorous regime should be implemented with lifestyle modification, physical activity, and cautious use of medications that lower blood glucose. Rather than intensifying diabetes control, a regimen tailored to the individual risk of autonomic imbalance should be implemented. New agents that may improve autonomic function, such as SGLT2 inhibitors, should be considered and the use of incretins monitored. One of the central mechanisms of dysfunction is disturbance of the hypothalamic cardiac clock, a consequence of dopamine deficiency that leads to sympathetic dominance, insulin resistance, and features of the metabolic syndrome. An improvement in ANS balance may be critical to reducing cardiovascular events, cardiac failure, and early mortality in the diabetic population.

AMERICAN ASSOCIATION OF CLINICAL ENDOCRINOLOGISTS AND AMERICAN COLLEGE OF ENDOCRINOLOGY POSITION STATEMENT ON TESTING FOR AUTONOMIC AND SOMATIC NERVE DYSFUNCTION

This document represents the official position of the American Association of Clinical Endocrinologists and the American College of Endocrinology. Where there were no randomized controlled trials or specific U.S. FDA labeling for issues in clinical practice, the participating clinical experts utilized their judgment and experience. Every effort was made to achieve consensus among the committee members. Position statements are meant to provide guidance, but they are not to be considered prescriptive for any individual patient and cannot replace the judgment of a clinician.

Predictability of successful trans-arterial embolization in pelvic fracture bleeding based on patient initial presentation

BACKGROUND

Pelvic fracture bleeding generally leads to hemorrhagic shock. Trans-arterial embolization (TAE) is regarded as the most useful treatment; however, the initial presentation of the patient can impact the effectiveness of TAE for pelvic fracture bleeding. The aim of this retrospective study is to explore whether the patient data at the initial presentation can predict the success of TAE for pelvic fracture bleeding.

METHODS

Twenty-seven charts were retrospectively reviewed. TAE failure was defined as any patient who eventually received an exigent laparotomy or who died due to uncontrolled bleeding after TAE. For patients who received TAE, we analyzed factors recorded at the initial presentation, including age, gender, systolic blood pressure, heart rate, respiratory rate, body temperature, Glasgow coma scale (GCS) score, injury severity score (ISS) and associated injuries, using Pearson's correlation and independent t-tests. The odds ratio was used to determine the cut-off values for the patient presentation findings related to successful TAE and thus was used to assess congruity.

RESULTS

Successful TAE was not correlated with age or gender. The hierarchical order of statistically significant associations between successful TAE and initial presentation data was as follows: the patient's body temperature, associated injury, respiratory rate, systolic blood pressure, GCS score, and ISS. The odds ratios for all statistically significant initial presentation factors were within a 95% confidence interval.

CONCLUSION

The findings upon initial presentation of a patient with pelvic fracture bleeding that were related to the predictability of successful TAE include the following: hypothermia prevention with maintenance of the body temperature above 36°C, associated injuries limited to two organ systems, maintenance of the respiratory rate at approximately twenty-two breaths per minute, a sustained systolic blood pressure of approximately 90mmHg, maintenance of a heart rate of approximately one hundred beats per minute, a minor head injury with a GCS score greater than thirteen and a moderate ISS of less than twenty.

Influence of music and its genres on respiratory rate and pupil diameter variations in cats under general anaesthesia: contribution to promoting patient safety

OBJECTIVES

The aims of the study were to recognise if there is any auditory sensory stimuli processing in cats under general anaesthesia, and to evaluate changes in respiratory rate (RR) and pupillary diameter (PD) in anaesthetised patients exposed to different music genres, while relating this to the depth of anaesthesia.

METHODS

A sample of 12 cats submitted for elective ovariohysterectomy was exposed to 2 min excerpts of three different music genres (classical [CM], pop [PM] and heavy metal [HM]) at three points during surgery (T1 = coeliotomy; T2 = ligature placement and transection of the ovarian pedicle; T3 = ligature placement and transection of the uterine body). A multiparametric medical monitor was used to measure the RR, and a digital calliper was used for PD measurement. Music was delivered through headphones, which fully covered the patient's ears. P values   <0.05 were considered to be statistically significant.   

RESULTS

Statistically significant differences between stimuli conditions for all surgical points were obtained for RR (T1, P = 0.03; T2, P = 0.00; T3, P = 0.00) and for PD (T1, P = 0.03; T2, P = 0.04; T3, P = 0.00). Most individuals exhibited lower values for RR and PD when exposed to CM, intermediate values to PM and higher values to HM.

CONCLUSIONS AND RELEVANCE

The results suggest that cats under general anaesthesia are likely to perform auditory sensory stimuli processing. The exposure to music induces RR and PD variations modulated by the genre of music and is associated with autonomic nervous system activity. The use of music in the surgical theatre may contribute to allowing a reduced anaesthetic dose, minimising undesirable side effects and thus promoting patient safety.

Using what you get: dynamic physiologic signatures of critical illness

The development and resolution of cardiopulmonary instability take time to become clinically apparent, and the treatments provided take time to have an impact. The characterization of dynamic changes in hemodynamic and metabolic variables is implicit in physiologic signatures. When primary variables are collected with high enough frequency to derive new variables, this data hierarchy can be used to develop physiologic signatures. The creation of physiologic signatures requires no new information; additional knowledge is extracted from data that already exist. It is possible to create physiologic signatures for each stage in the process of clinical decompensation and recovery to improve outcomes.

Cardiac autonomic testing and diagnosing heart disease. "A clinical perspective"

Background

Coronary heart disease (CHD) is a major health concern, affecting nearly half the middle-age population and responsible for nearly one-third of all deaths. Clinicians have responsibilities beyond diagnosing CHD, including risk stratification of patients for major adverse cardiac events (MACE), modifying the risks and treating the patient. In this first of a two-part review, identifying risk factors is reviewed, including more potential benefit from autonomic testing.

Methods

Traditional and non-traditional, and modifiable and non-modifiable risk factors for MACE where compared, including newer risk factors, such as inflammation, carotid intimal thickening, ankle-brachial index, CT calcium scoring, and autonomic function testing, specifically independent measurement of parasympathetic and sympathetic (P&S) activity.

Results

The Framingham Heart Study, and others, have identified traditional risk factors for the development of CHD. These factors effectively target high-risk patients, but a large number of individuals who will develop CHD and MACE are not identified. Many patients with CHD who appear to be well-managed by traditional therapies still experience MACE. In order to identify these patients, other possible risk factors have been explored. Advanced autonomic dysfunction, and its more severe form, cardiac autonomic neuropathy, have been strongly associated with an elevated risk of cardiac mortality and are diagnosable through P&S testing.

Conclusions

Independent measures of P&S activity, provides additional information and has the potential to incrementally add to risk assessment. This additional information enables physicians to (1) specifically target more high-risk patients and (2) titrate therapies, with autonomic testing guidance, in order to minimize risk of cardiac mortality and morbidity.

Comparison of heart and respiratory rate variability measures using an intermittent incremental submaximal exercise model

To better understand the alterations in cardiorespiratory variability during exercise, the present study characterized the patterns of change in heart rate variability (HRV), respiratory rate variability (RRV), and combined cardiorespiratory variability (HRV-RRV) during an intermittent incremental submaximal exercise model. Six males and six females completed a submaximal exercise protocol consisting of an initial baseline resting period followed by three 10-min bouts of exercise at 20%, 40%, and 60% of maximal aerobic capacity (V̇O2max). The R-R interval and interbreath interval variability were measured at baseline rest and throughout the submaximal exercise. A group of 93 HRV, 83 RRV, and 28 HRV-RRV measures of variability were tracked over time through a windowed analysis using a 5-min window size and 30-s window step. A total of 91 HRV measures were able to detect the presence of exercise, whereas only 46 RRV and 3 HRV-RRV measures were able to detect the same stimulus. Moreover, there was a loss of overall HRV and RRV, loss of complexity of HRV and RRV, and loss of parasympathetic modulation of HRV (up to 40% V̇O2max) with exercise. Conflicting changes in scale-invariant structure of HRV and RRV with increases in exercise intensity were also observed. In summary, in this simultaneous evaluation of HRV and RRV, we found more consistent changes across HRV metrics compared with RRV and HRV-RRV.

Heart rate variability is an independent predictor of morbidity and mortality in hemodynamically stable trauma patients

BACKGROUND

Reduced heart rate variability (HRV) reflects autonomic dysfunction and can triage patients better than routine trauma criteria or vital signs. However, there is questionable specificity and no consensus measurement technique. The purpose of this study was to analyze whether factors that alter autonomic function affect the specificity of HRV for assessing traumatic injury.

METHODS

We evaluated 216 hemodynamically stable adults (3:1 M:F; 97:3 blunt:penetrating; age 49 years ± 1 year, mean ± standard error) undergoing computed axial tomography (CT) scan to rule out traumatic brain injury (TBI). All were prospectively instrumented with a Mars Holter system (GE Healthcare, Milwaukee, WI). HRV was determined offline using time domain (standard deviation of normal-normal intervals, root-mean-square successive difference) and frequency domain (very low frequency [VLF], LF, wideband frequency, high frequency [HF], low to HF index ratio) calculations from 15-minute electrocardiogram and correlated with routine vital signs, mortality, TBI, morbidity, length of stay (LOS), and comorbidities. Significance (p ≤ 0.05) was determined using nonparametric analysis, Student's t test, analysis of variance, or multiple logistic regression.

RESULTS

VLF alone predicted survival, severity of TBI, intensive care unit LOS, and hospital LOS (all p < 0.05). Beta-blockers or diabetes had no effect, whereas age, sedation, mechanical ventilation, spinal cord injury, and intoxication influenced one or more of the variables with age being the most powerful confounder (all p < 0.05). Except for the Glasgow Coma Scale, no other routine trauma or hemodynamic criteria correlated with any of these outcomes.

CONCLUSIONS

Decreased VLF is an independent predictor of mortality and morbidity in hemodynamically stable trauma patients. Other time and other frequency domain variables correlated with some, but not all, outcomes. All were heavily influenced by factors that alter autonomic function, especially patient age.

Impaired blood pressure recovery to hemorrhage in obese Zucker rats with orthopedic trauma

We have shown that obese Zucker rats with orthopedic trauma (OZT) exhibit a loss of arteriolar tone in skeletal muscle. We hypothesize that the loss of arteriolar tone in OZT blunts vasoconstrictor responses to hemorrhage, resulting in an impaired blood pressure recovery. Orthopedic trauma was induced with soft tissue injury and local injection of bone components in both hindlimbs in lean (LZT) and OZT (11-13 wk). One day after the orthopedic trauma, blood pressure responses following hemorrhage were measured in conscious control lean, control obese, LZT, and OZT. In another set of experiments, the spinotrapezius muscle of control and trauma animals was prepared for microcirculatory observation. Arteriolar responses to phenylephrine (PE) or hemorrhage were determined. Hemorrhage resulted in similar blood pressure responses in control animals and LZT, but the blood pressure recovery following hemorrhage was blunted in the OZT. In the spinotrapezius, OZT exhibited decreased arteriolar tone and blunted vasoconstrictor responses to PE and hemorrhage. Treatment with glibenclamide improved the blood pressure recovery in the conscious OZT and improved the arteriolar tone, and PE induced vasoconstriction in the spinotrapezius of the OZT. Thus, ATP-dependent K(+) channel-mediated loss of arteriolar tone in OZT blunts the arteriolar constriction to hemorrhage, resulting in impaired blood pressure recovery.

Core temperature variation is associated with heart rate variability independent of cardiac index: a study of 278 trauma patients

PURPOSE

The purpose of this study is to determine if temperature extremes are associated with reduced heart rate variability (HRV) and "cardiac uncoupling."

MATERIALS AND METHODS

This was a retrospective, observational cohort study performed on 278 trauma intensive care unit admissions that had continuous HR, cardiac index (CI), and core temperature data from "thermodilution" Swan-Ganz catheter. Dense (captured second-by-second) physiologic data were divided into 5-minute intervals (N = 136 133; 11 344 hours of data). Mean CI, mean temperature, and integer HR SD were computed for each interval. Critically low HRV was defined as HR SD from 0.3 to 0.6 beats per minute. Temperature extremes were defined as less than 36°C or greater than 39°C.

RESULTS

Low HRV and CI vary with temperature. Temperature extremes are associated with increased risk for critically low HRV (odds ratio, >1.8). Cardiac index increases with temperature until hyperthermia (>40°C). At temperature extremes, changes in CI were not explained solely by changes in HR.

CONCLUSIONS

The conclusions of this study are (1) temperature extremes are associated with low HRV, potentially reflecting cardiac autonomic dysfunction; (2) CI increases with temperature; and (3) HRV provides additional physiologic information unobtainable via current invasive cardiac monitoring and current vital signs.

Clinical applications of heart rate variability in the triage and assessment of traumatically injured patients

Heart rate variability (HRV) is a method of physiologic assessment which uses fluctuations in the RR intervals to evaluate modulation of the heart rate by the autonomic nervous system (ANS). Decreased variability has been studied as a marker of increased pathology and a predictor of morbidity and mortality in multiple medical disciplines. HRV is potentially useful in trauma as a tool for prehospital triage, initial patient assessment, and continuous monitoring of critically injured patients. However, several technical limitations and a lack of standardized values have inhibited its clinical implementation in trauma. The purpose of this paper is to describe the three analytical methods (time domain, frequency domain, and entropy) and specific clinical populations that have been evaluated in trauma patients and to identify key issues regarding HRV that must be explored if it is to be widely adopted for the assessment of trauma patients.

Positive end-expiratory pressure may alter breathing cardiovascular variability and baroreflex gain in mechanically ventilated patients

Background

Baroreflex allows to reduce sudden rises or falls of arterial pressure through parallel RR interval fluctuations induced by autonomic nervous system. During spontaneous breathing, the application of positive end-expiratory pressure (PEEP) may affect the autonomic nervous system, as suggested by changes in baroreflex efficiency and RR variability. During mechanical ventilation, some patients have stable cardiorespiratory phase difference and high-frequency amplitude of RR variability (HF-RR amplitude) over time and others do not. Our first hypothesis was that a steady pattern could be associated with reduced baroreflex sensitivity and HF-RR amplitude, reflecting a blunted autonomic nervous function. Our second hypothesis was that PEEP, widely used in critical care patients, could affect their autonomic function, promoting both steady pattern and reduced baroreflex sensitivity.

Methods

We tested the effect of increasing PEEP from 5 to 10 cm H2O on the breathing variability of arterial pressure and RR intervals, and on the baroreflex. Invasive arterial pressure, ECG and ventilatory flow were recorded in 23 mechanically ventilated patients during 15 minutes for both PEEP levels. HF amplitude of RR and systolic blood pressure (SBP) time series and HF phase differences between RR, SBP and ventilatory signals were continuously computed by complex demodulation. Cross-spectral analysis was used to assess the coherence and gain functions between RR and SBP, yielding baroreflex-sensitivity indices.

Results

At PEEP 10, the 12 patients with a stable pattern had lower baroreflex gain and HF-RR amplitude of variability than the 11 other patients. Increasing PEEP was generally associated with a decreased baroreflex gain and a greater stability of HF-RR amplitude and cardiorespiratory phase difference. Four patients who exhibited a variable pattern at PEEP 5 became stable at PEEP 10. At PEEP 10, a stable pattern was associated with higher organ failure score and catecholamine dosage.

Conclusions

During mechanical ventilation, stable HF-RR amplitude and cardiorespiratory phase difference over time reflect a blunted autonomic nervous function which might worsen as PEEP increases.

Noninvasive monitoring of the autonomic nervous system and hemodynamics of patients with blunt and penetrating trauma

BACKGROUND

To describe early effects of sympathetic (SNS) and parasympathetic nervous system (PSNS) activities measured by heart rate (HR) and respiratory rate variabilities simultaneously with noninvasive hemodynamic patterns in patients with blunt and penetrating trauma.

METHODS

Descriptive study of 168 monitored trauma patients in a level I university-run trauma service. We studied HR and respiratory rate variability by spectral analysis as a measure of autonomic nervous system (ANS) activity in severe blunt and penetrating injuries beginning shortly after their admission to the emergency department. The low frequency area is the area under the HR spectral analysis curve within the frequency range of 0.04 Hz to 0.10 Hz. This area primarily reflects the tone of the SNS as mediated by the vagus nerve. The respiratory frequency area, sometimes referred to as the high frequency area, is a 0.12 Hz-wide frequency range centered around the fundamental respiratory frequency defined by the peak mode of the respiratory activity power spectrum. It is indicative of vagal outflow reflecting PSNS activity. The low frequency area/respiratory frequency area, or L/R ratio, reflects the balance of the SNS and the PSNS. ANS was studied simultaneously with noninvasive hemodynamic patterns after blunt and penetrating thoracic or abdominal injury beginning shortly after admission. We measured cardiac index by bioimpedance, HR, and mean arterial pressure (MAP) to evaluate cardiac function, pulse oximetry (SapO2) to reflect changes in respiratory function, and transcutaneous oxygen indexed to fractional inspired oxygen (PtcO2/FIO2) to reflect tissue perfusion.

RESULTS

ANS activity markedly increased especially in the nonsurvivors at 12 hours to 24 hours after admission. Compared with survivors, the nonsurvivors had lower MAP, CI, and PtcO2/FIO2 values associated with increased ANS activity.

CONCLUSIONS

In the nonsurvivors, low flow, low MAP, and reduced tissue perfusion were associated with pronounced increases in PSNS and lesser increases in SNS activity. In the survivors, higher CI, MAP, and PtcO2/FIO2 values were associated with lesser increases in both PSNS and SNS activities.

An observational, prospective study exploring the use of heart rate variability as a predictor of clinical outcomes in pre-hospital ambulance patients

OBJECTIVE

To explore the use of pre-hospital heart rate variability (HRV) as a predictor of clinical outcomes such as hospital admission, intensive care unit (ICU) admission and mortality. We also implemented an automated pre-analysis signal processing algorithm and multiple principal component analysis (PCA) for outcomes.

MATERIALS AND METHODS

We conducted a prospective observational clinical study at an emergency medical services (EMS) system in a medium sized urban setting in the United States. Electrocardiogram (ECG) data was obtained from a sample of 45 ambulance patients conveyed to a tertiary hospital, monitored with a LIFEPAK12 defibrillator/monitor. After extracting the data, filtering for noise reduction and isolating non-sinus beats, various HRV parameters were computed. These included time domain, frequency domain and geometric parameters. PCA was performed on the hospital outcomes for these patients.

RESULTS

We used a combination of HRV parameters, age and vital signs such as respiratory rate, SpO2 and Glasgow coma score (GCS) in a PCA analysis. For predicting admission to ICU, sensitivity was 100%, specificity was 48.6%, and negative predictive value (NPV) was 100%; for predicting admission to hospital, sensitivity was 78.9%, specificity was 85.7%, and NPV was 75.0%; for predicting death, sensitivity was 50.0%, specificity was 100%, and NPV was 97.4%. There was also a significant correlation of several HRV parameters with length of hospital stay.

CONCLUSIONS

With signal processing techniques, it is feasible to filter and analyze ambulance ECG data for HRV. We found a combination of HRV parameters and traditional 'vital signs' to have an association with clinical outcomes in pre-hospital patients. This may have potential as a triage tool for ambulance patients.

Beta-blocker exposure in patients with severe traumatic brain injury (TBI) and cardiac uncoupling

BACKGROUND

Cardiac uncoupling and reduced heart rate (HR) variability are associated with increased mortality after severe traumatic brain injury (TBI). Recent data has shown beta-blocker (betaB) exposure is associated with improved survival in this patient population. The purpose of the present study was to evaluate the effect of betaB exposure on the mortality risk of patients with severe TBI and early cardiac uncoupling.

METHODS

From December 2000 to October 2005, 4,116 patients were admitted to the trauma intensive care unit. Four hundred forty-six patients (12%) had head Abbreviated Injury Scale score >/= 5 without neck injury and had continuous HR data for the first 24 hours. One hundred forty-one patients (29%) received betaB. Cardiac uncoupling was calculated as the percent of time that 5-minute HR standard deviation was between 0.3 bpm and 0.6 bpm on postinjury day 1.

RESULTS

A relationship between betaB and survival was observed when the population was considered irrespective of length of stay or betaB start time (p < 0.001). Cardiac uncoupling appears to stratify patients into groups who might receive additional benefit from betaB, and identifies patients with increasing mortality. However, the association of betaB with survival was attenuated when analyses accounted for selection bias in betaB administration.

CONCLUSIONS

betaB exposure was associated with reduced mortality among patients with severe TBI. Though loss of HR variability has previously been associated with an increase in mortality, betaB exposure appears to be associated with increased survival across all stratifications of cardiac uncoupling.

Heart rate variability index in trauma patients

BACKGROUND

Heart rate variability (HRV) changes often reflect autonomic dysfunction with high sensitivity, but the specificity is also low. There are several different methods for measuring HRV, but interpretation is often complex, and the units are not interchangeable. For these reasons, HRV monitoring is not routinely used in many clinical situations. We hypothesized that the specificity of HRV as a screening tool for trauma patients could be improved by controlling some of the confounding influences using multiple logistic regression.

METHODS

A prospective observational trial with waiver of consent was performed in 243 healthy student volunteers and 257 trauma patients, in the resuscitation bay and intensive care units of a Level I trauma center, who received computed axial tomography (CT) scans of the head as part of the initial work up. Electrocardiogram results were recorded for 5 minutes. HRV was defined by SD of normal R-R intervals (SDNN5) and by root mean square of successive differences of R-R intervals (RMSSD5). A head CT scan was considered positive (+) if there were abnormalities in the parenchyma (diffuse axonal injury or contusion), vasculature (intraparenchymal, subdural, or epidural hemorrhage), and/or structural or bony components (fractures of the face or cranium).

RESULTS

In volunteers, SDNN5 was 73 +/- 15 (M +/- SD) milliseconds, compared with 42 +/- 22, 31 +/- 19, 28 +/- 17, and 12 +/- 8 milliseconds in, CT(-) patients with no sedation (n = 82), CT(-) with sedation (n = 60), CT(+) with no sedation (n = 55), and CT(+) with sedation (n = 60), respectively. The differences between trauma, sedation, and CT categories were significant (all p < 0.001). RMSSD5 differences were similar and also highly significant (all p < 0.001). For both SDNN5 and RMSSD5, in each category, there was wide overlap in the range of values, and strong inverse correlations with heart rate (all p < 0.001). Using multiple logistic regression in a subset with no missing data (n = 194), an index was derived from ln(SDNN5) adjusted for six confounding factors. With a negative predictive value held constant at 0.90, compared with ln(SDNN5) alone, the stepwise addition of heart rate, sedation, age, gender, and blood pressure progressively improved the specificity of the HRV index from 0.56 to 0.77, positive predictive value from 0.55 to 0.68, and efficiency from 0.68 to 0.80. This index was then normalized (0-100 scale) for ease of interpretation.

CONCLUSIONS

(1) Several factors alter HRV in patients; (2) when HRV was indexed for some of these factors, its specificity and efficiency were improved for predicting a discrete pathologic state in trauma patients, i.e. (+) or (-) cranial CT scans; (3) the algorithm can incorporate other factors to further refine the diagnostic and/or prognostic ability of HRV as a noninvasive clinical tool; (4) this concept should be applicable to any other HRV measurement technique or outcome.

Smoothed Periodogram of Oxyhemoglobin Saturation by Pulse Oximetry in Sleep Apnea Syndrome

BACKGROUND

Variability of oxyhemoglobin saturation (Spo(2)) during sleep has been utilized as a diagnostic index for sleep apnea. Spectral analysis with its graphical presentation, the periodogram, is an approach for measuring such variability. This work examined the parameters on a smoothed periodogram created from series data for Spo(2) obtained by pulse oximetry during a sleep study.

DESIGN AND RESULTS

Spo(2) was recorded during polysomnography study of 273 subjects. Clinical data of subjects were collected retrospectively. A novel automated algorithm was created to measure the low-frequency (< 0.1 Hz) peak and the slope of spectral density vs frequency in the frequency region of 0.1 to 0.5 Hz (slope(0.1-0.5)). Two successive modified Daniell smoothers with span lengths of 3 to 121 in odd numbers were applied to determine the effect of smoothing on these parameters. slope(0.1-0.5) was least affected by smoothing and had a sensitivity of 78% and a specificity of 80% in diagnosing sleep apnea defined by a value of apnea-hypopnea index >/= 5. Combining slope(0.1-0.5) with parameters of the low-frequency peak enlarged the area under the receiver operating characteristic curve. A composite indicator comprised of slope(0.1-0.5) and ratio of the area under the curve of the low-frequency peak to that of whole periodogram (AUCratio) had a positive likelihood ratio of 15.25 in identifying patients with moderate-to-severe obstructive sleep apnea. The algorithm was validated in another 206 patients undergoing polysomnographic studies.

CONCLUSIONS

These analytical results demonstrate that the smoothed periodogram of Spo(2) is a useful tool for screening subjects with sleep apnea.

Complex systems and the technology of variability analysis

Characteristic patterns of variation over time, namely rhythms, represent a defining feature of complex systems, one that is synonymous with life. Despite the intrinsic dynamic, interdependent and nonlinear relationships of their parts, complex biological systems exhibit robust systemic stability. Applied to critical care, it is the systemic properties of the host response to a physiological insult that manifest as health or illness and determine outcome in our patients. Variability analysis provides a novel technology with which to evaluate the overall properties of a complex system. This review highlights the means by which we scientifically measure variation, including analyses of overall variation (time domain analysis, frequency distribution, spectral power), frequency contribution (spectral analysis), scale invariant (fractal) behaviour (detrended fluctuation and power law analysis) and regularity (approximate and multiscale entropy). Each technique is presented with a definition, interpretation, clinical application, advantages, limitations and summary of its calculation. The ubiquitous association between altered variability and illness is highlighted, followed by an analysis of how variability analysis may significantly improve prognostication of severity of illness and guide therapeutic intervention in critically ill patients.